This invention relates to x-ray devices and more particularly, to a low fluorescence, low back scatter x-ray beam stop. Such a beam stop is employed with x-ray devices particularly medical diagnostic x-ray devices in connection with their x-ray imaging detectors and x-ray control detectors to absorb primary radiation from the x-ray source, and to minimize secondary radiation from the beam stop from producing spurious signals in the detectors.
Medical diagnostic x-ray imaging devices are equipped with an x-ray image detector which forms an image in an x-ray sensitive medium after the x-ray beam passes through the object to be examined. The x-ray device may further include an x-ray control detector through which the x-ray beam also passes. An x-ray control detector is utilized to provide an electrical signal correlated to the exposure or intensity of the passing x-ray beam. By this means an operator may more correctly correlate the quality of an image obtained in the device with the x-ray exposure or intensity utilized, and adjust the device to provide a different or better image or to minimize undesirable x-ray radiation. An x-ray beam stop is employed in conjunction with a detector to absorb the x-ray beam after the beam has passed through the object to be examined and the image and control detectors. The beam stop effectively absorbs x-rays transmitted by the noted detectors to minimize undesirable radiation near the x-ray device where, in the medical x-ray field, operators, patients, and the diagnostic device's sensitive electronic control systems may be exposed to potentially harmful radiation. Furthermore, the beam stop may enable an x-ray detector to function more accurately for x-ray adjustment purposes to reduce the level of x-ray radiation to only that necessary for a given purpose.
X-ray imaging detectors may include intensifying screen film detectors, photo stimulatable phosphor detectors, as well as solid state electronic detectors. Image detectors may be deleteriously affected by secondary radiation from a closely adjacent beam stop device which can penetrate the detector and degrade the image by producing artifactual signals or reducing image contrast. An x-ray control detector may comprise a grid structure defining predetermined zones or volumes in which an electron emitter is impinged by the x-ray beam to generate high energy electrons. The electrons ionize a gas in the defined zones and an appropriately electrically biased collection electrodes in the zone collects the ions to generate resultant voltage which is amplified to produce a signal correlated to x-ray intensity. An x-ray beam stop closely adjacent a detector, as described, absorbs and stops the x-ray beam. However, any secondary x-ray radiation from the beam stop penetrating into the control detector could be absorbed therein and produce a deleterious signal or adversely affect sensitive electronic components in the detector.
Lead (Pb) is a primary absorption medium employed in x-ray beam stops where it has been found to be significantly effective. As with other metals, when lead is impinged by an x-ray beam, it tends to give off secondary x-ray radiation the strength of which is dependent on the energy of the incident x-ray beam striking the lead medium. If the energy of the incident x-ray is sufficiently large, it may remove an electron from an inner one of the plural spaced electron shells surrounding a metal atom with the result that an electron from an outer shell will move into the position vacated by the removed electron. In so doing, considerable energy is released which may take the form of radiation referred to as k-fluorescence radiation. The material in which the k-fluorescence is created is relatively transparent to this radiation resulting in a large fraction passing out of the material. For example, x-rays with energies greater than 88 keV (kilo electron volts) are likely to produce k-fluorescent x-rays from lead with an average energy of 77 keV which enables a large fraction of k fluorescence x-rays to escape the lead to enter an adjacent x-ray detector (image or control) and produce undesirable signals.